Measurement of conversion conductance



PATENT omen MEASUREMENT OF CONVERSION CON- DUCTANCE Komnenus M. Soukaras, Washington, D. 0.

Application April 26, 1939, Serial No. 270,265

2 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) This invention relates to the measurement of conversion conductance of a frequency converter vacuum tube, and more particularly, to the measurement of that quantity at power frequencies.

Among the several objects of this invention are:

To .provide simple and accurate means for measuring, at usual power frequencies, the conversion conductance of a converter or mixer tube;

To eliminate the complications involved in determining conversion conductance at radio frequencies or by mathematical calculation from static characteristics.

Other objects will appear when the following description is read in connection with the drawing wherein:

Fig. 1 is a general schematic diagram of a frequency converter tube and the immediately associated circuits;

Fig. 2 depicts a circuit embodying the present invention.

Conversion conductance is defined by the Standards Committee of the Institute of Radio Engineers as the quotient of the magnitude of a single beat-frequency component (f1+f2) or 1f2) of the output-electrode current by the magnitude of the control-electrode voltage of frequency f1, under the conditions that all electrode voltages and the magnitude of the electrode alternating voltage 12 remainconstant and that no impedance at the frequency f1 or f2 is present in the output circuit. As most precisely used, the term refers to an infinitesimal magnitude of the voltage of frequency f1.

Conversion conductance may be obtained mathematically from the static characteristics, or at radio frequencies. The first method requires elaborate mathematical treatment, and the second method elaborate apparatus. Both methods are time consuming.

In what follows will be described a simplified method for measuring conversion conductance at power frequencies by means of a separately excited milliammeter of the dynamometer type.

Consider a frequency converter tube of the type shown in Fig. 1. Let two sinusoidal voltages of frequencies f1 and f2, respectively, be applied to the signal and oscillator grids as shown.

Let

61=E1 sin (a t f signal frequency f oscillator frequency. A180, let E1 E2.

The signal frequency wave may be represented as modulating the local frequency wave. For an amplitude modulated wave we have the following well known relationship:

cos (cor-cog)! E2 COS wzt going through any impedance load. Then in terms of currents, instead of voltages, Equation 2 may be written as where again, E1 is the peak value of the control signal voltage, and 11.2 is also an amplitudeor peak value.

Substituting (3) in (4) we have,

If E1 E2, it is evident that In terms of currents we have Substituting (6) in (5) Equation 7 is the equation for conversion conductance when both E1 and E2 are actingtogether. However, if we make E1, the amplitude of the control signal voltage, zero, and compensate for it by a variation of the control signal grid bias, that is by the validity of the resulting equation is not impaired.

We have finally,

1 (A1 peak In words, Equation 8 reads:

The conversion conductance is equal to onehalf the change in peak value of the fundamental component of the output electrode current divided by the change in potential of the signal electrode.

The circuit actually embodying the present invention is depicted in Fig. 2. Transformer 3 supplies 60 cycle alternating current to the circuit comprising variable resistor l,'resistor 5 and field coil 6 of a milliammeter of the dynamometer type. It is apparent that current from the secondary l of transformer 3 will flow through resister 5 and field coil 6 and that the potential drop across resistor 5 will necessarily be in phase with the current through field coil 6, since that potential drop will be produced by the flow of current. Oscillator grid 8 of converter tube 9 is connected to such point on resistor 5 as to impress on the grid 8 a voltage substantially the same as will be carried by grid 8 in actual operation. This is determined by measuring the fio-w of rectified oscillator grid current by means of microammeter ill in the circuit between oscillator grid 8 and cathode H. Resistor I2 may be used in the circuit of oscillator grid 8 to effect biasing if desired.

Signal grid E3 of tube 9 has applied to it a voltage derived from battery I4 by way of volt-- age divider l5. Moving coil [6 of the milliammeter is in series with anode ll of tube 9 and since the plate current is in phase with the oscillator voltage applied to grid 8 and this voltage is in turn in phase with the current through field coil 6, it follows that the currents in field coil 6 and moving coil it are in phase with each other. It is to be understood that coils 6 and it are mutually coupled magnetically only and that there exists no direct conductive electrical connection between these coils such as to pass conductive current from one to the other. Capacitances l8 provide low impedance paths between the various electrode elements (except grid I3) of tube 9 and the cathode at the frequency employed on oscillator grid 8.

The method of measurement is as follows:

A voltage is applied to signal grid 53 by means of voltage divider l5 and the rectified grid current through oscillator grid 8 is brought to the proper value by adjusting the connection of grid 8 to resistor 5. The reading of voltmeter l9 and of the milliammeter having the operating elements field coil 6 and-moving coil it are taken.

The Voltage on grid I3 is then varied slightly by an adjustment of voltage divider l5, voltmeter I9 is read, and the value of anode current corresponding to the second voltage on grid I3 is read from the milliammeter. It is then merely a matter of inserting the values of the current.

1. The combination with a frequency converter tube having a cathode, an anode and a plurality of grids including a signal grid and an oscillator grid, of means for supplying a low frequency current, current measuring means of the dynamometer type having a field coil and a moving coil electrically conductively isolated from each other and having mutual magnetic coupling only, a resistance and said field coil connected in series with said means, means adjustably connecting said oscillator grid to said resistance to apply a selected voltage to said oscillator grid, means to apply a'steady voltage to said signal grid and to vary said voltage in increments, means to measure said steady voltage, means connecting said moving coil in series with said anode, and means constituting low impedance paths at said frequency between said cathode and each ofrsaid grids except said signal grid. v

2. The combination with a vacuum tube having an anode, a cathode and multiple grids, of a current measuring instrument having a fixed and a movable operating element, said elements being 1 operatively associated by mutual inductive coupling only, a series circuit carrying low frequency current and including one of said elements and means to produce a potential difference in phase withthe current through said one element, means to apply to one of said grids a selected. potential derived from the aioresaid'means, means to ap: ply to a second said grid a steady potential selectivelyvariable in increments, means to meas- -KOMNENUS M. SOUKARAS'. 

